This section provides background information related to the present disclosure which is not necessarily prior art.
Multi-output power converters are used in numerous applications including power supplies for desktop computers, workstation computers and LCD televisions. In recent years, high efficiency requirements for power supplies and power converters have been dictated by various governmental agencies, such as the EPA, and non-governmental voluntary agencies, such as Climate Savers. Although target power supply efficiencies may be achieved, it often results in a very high cost premium and/or low power densities.
For computer power supply applications, the load profile is well known and the minimum load on each output can be easily defined. The expected load variation has a predictable profile. The typical output voltages in a multi-output computer power supply are 3.3V, 5V and 12V, with the bulk of the power on the 12V rail. The slew rate of the transient step load on each output of the power supply is not very demanding and is typically about 0.1 A/uSec. The regulation of each output voltage typically needs to be in the range of +/−5%.
LCD television power supplies typically provide three to four output voltages, such as +12V, −12V and 24V. Such power supplies often require galvanic isolation between the multiple outputs to achieve a common chassis ground and avoid circulating currents. The loads on the multiple output rails can vary between no load and full load, including a short term peak load rating. For example, the loading on a power supply output used for an audio amplifier can decrease to near no load when the sound is muted. Similarly, the load on the output providing power to an LCD inverter can swing from no load to full load when the picture frame switches between completely dark to fully bright. Such wide load ranges on each output can exist simultaneously, making it difficult for a simple converter with a coupled winding to achieve cross regulation.
Several known techniques meet these application requirements but struggle to attain high efficiency while being cost effective. A few of the established techniques are discussed below.
In one known power supply, a standard forward converter is used in conjunction with magnetic amplifiers to deliver multiple regulated outputs. Typically, one output is directly regulated by the control loop of the forward converter while magnetic amplifiers are used for the other outputs. Alternatively, magnetic amplifiers can be used for all the outputs as well. Low conversion efficiency and electromagnetic noise due to interaction between different magnetic amplifiers are common difficulties with such an approach.
In a second approach, independent power converters are used for each output rail. Although this can achieve desired efficiency goals, it uses a large number of components and can be expensive.
Another known approach is to use a high efficiency converter with synchronous rectification for the main, e.g., 12V, output. The main output is then used as an input bus for separate high efficiency buck converters using synchronous rectifiers. These separate buck converters are driven by the input bus to generate low voltage outputs. Such an approach may be costly and can achieve only moderate efficiencies as the power losses are cascaded in series.
Another technique uses multiple secondary windings on a single transformer. The main output, usually the 12V output, utilizes closed loop control to maintain a well regulated output. The other low voltage outputs, such as 3.3V and 5V, depend upon inherent cross regulation. Minor trimming of these low voltage outputs is achieved by dropping some voltage across the synchronous rectifiers by operating them in linear mode. Thus the synchronous rectifiers are operated similar to a variable resistor. This method is dissipative and gives little control over regulation as the voltage drop in a synchronous rectifier can only be controlled in the range of 0V to 0.7V. Additionally, this method tends to lead to use of bulky and lossy transformers to meet design requirements for a multiple output power supply as the turns ratio must be the same as the ratio of output voltages. For example, a typical computer power supply having 3.3V, 5V and 12V outputs must have a transformer using 2, 3 and 7 secondary turns, respectively, or an integer multiple. It is also very difficult to extend the operation of such a power supply to higher power applications, such as workstation computers which may need up to 1 kW of output power.